To get to entrance to Forsaken City enter tunnel at ( -265, -7, 53 ) /waypoint -265, -7, 53 in the Mystic Lake zone in the Isle of Mara.
Underwater breathing is recommended but not neccessary if you know the way.

Follow stairs down to room with doors to the north and south and go north (right).

At doorway where you can go forward (west) or move right and go down stairs; take the stairs on your right ( -276, -22, 29 ) /waypoint -276, -22, 29

At the bottom of the stairs you enter a room with two doors south (left) and north (right). Turn north (right) and go down stairs (the room on the left is a dead end). These stairs take you to the entrance to the Forsaken City.( -264, -42, 49 ) /waypoint -264, -42, 49